In the following program, the initializer of the constexpr pointer p has undefined behavior.  As a result, the initializer is not a /core constant expression/ which makes the program ill-formed and subject to a requirement for a diagnostic.  However, GCC 6.0 and all supported versions before it that implement constexpr silently accept it.

$ cat z.c && /home/msebor/build/gcc-trunk-svn/gcc/xg++ -B/home/msebor/build/gcc-trunk-svn/gcc -S z.c
constexpr int a [] = { 1 };
constexpr const int *p = a - 1;

The EDG eccp front end rejects the program with the following output:

"z.c", line 2: warning: pointer points outside of underlying object
  constexpr const int *p = a - 1;
                             ^

"z.c", line 2: error: expression must have a constant value
  constexpr const int *p = a - 1;
                             ^

"z.c", line 2: warning: variable "p" was declared but never referenced
  constexpr const int *p = a - 1;
                       ^

1 error detected in the compilation of "z.c".